In many communications systems space is at a premium and therefore efforts are made to make antennas as compact as possible, while retaining adequate performance characteristics. In point-to-multipoint (PMP) microwave radio links especially, flat antennas are often installed in the terminal units due to their compact design. They can be easily integrated into boxes containing the electrical equipment of the outdoor units without detracting from the quality of the urban environment. For medium-gain requirements printed antennas are preferred. These have an upper gain limit of about 30 dB, due to the fact that the conductor losses in the associated feed networks increase considerably with antenna size. An alternative solution for higher gain are waveguide slot arrays, which have low losses but higher production costs. Hybrid configurations are also feasible using a mixed design with microstrip subarrays and a central waveguide feed network. In the case of dual polarization either a stacked design or two single polarized antennas side-by-side are necessary. All these antennas are more complicated than the simple printed array and require additional volume and thickness which is further increased by the presence of the radome, a flat dielectric plate placed a distance of approximately one wavelength above the antenna parallel to the array surface.
Examples are given in the existing literature of flat or parabolic reflectors with parallel metallic rings placed λ/4 above a metallic surface (zone-plate antennas)—see, for example, L. F. van Buskirk and C. E. Hend, “The Zone Plate as a Radio-Frequency Focusing Element”, IRE Transactions on Antennas and Propagation, vol. AP-9, No. 3, May 1961, pp 319-320; P. Cousin, G. Landrac, S. Toutain and J. J. Delmas, “Calcul de la Distribution de Champ Focal et du Diagramme de Rayonnement d'une Antenne Parabolique a Zones de Fresnel”, Journees Internationales de Nice sur les Antennes, Nice, November 1994, pp 489-492; Y. J. Guo, S. K. Barton, “Analysis of One-Dimensional Zonal Reflectors”, IEEE Transactions on Antennas and Propagation, vol. AP-43, No. 4, April 1995, pp 385-389. Also printed flat reflectors are known from, e.g., Y. J. Guo and S. K. Barton, “A High-Efficiency Quarter-Wave Zone-Plate Reflector”, IEEE Microwave and Guided-Wave Letters, vol. 2, No. 12, December 1992, pp 470-471.
A further example, which is illustrated in FIG. 1, involves the use of a parabolic reflector 10 in association with a subreflector 11, a dielectric cone 12 and a waveguide feed-section 13. In use signals to be transmitted from the antenna are fed into the waveguide 13 at the apex 14 of the reflector, are propagated along the waveguide and are carried through the dielectric cone 12 to the reflecting surface 15 of the subreflector 11, where they are reflected through the dielectric of the cone 12 onto the inner surface of the main reflector 10, being finally reflected from that surface out into free space in the same direction as the initial feed wave entering the apex 14. The dielectric cone 12 helps to ensure a correct illumination pattern on the main reflector 10. A step-transformer 16 may also be included in order to minimize unwanted back-reflections along the waveguide 13.
Two further aspects of this known design result in a considerable thickness of the entire antenna in the plane of the page. Firstly, a radome 17 is included, which is necessarily spaced a certain distance away from the main reflector 10—i.e. by at least λ/2 where a planar array is concerned. (The example shown in FIG. 1 is intended for point-to-point links, which have to meet more severe restrictions of the radiated power in large angular regions than a terminal antenna in a PMP application. This is achieved with the aid of a deep rim whose inner surface is coated with absorbing material. Consequently the very large distance of the radome from the reflector in FIG. 1 would not be required in the PMP setting currently being considered).
Secondly, the focal length of the reflector 10 requires that the subreflector 11 be placed that same distance away from the apex 14, having as a further consequence the considerable length of the feed-waveguide 13. As a result, therefore, the thickness of the entire antenna amounts to approximately 16λ (assuming an operating frequency of around 32 GHz). Furthermore, the great length of the waveguide may increase the overall return-losses in a broadband system.